When I bought a tatty top box to refurbish, one of the hooks on the top box lid, that the lock latches into, had snapped. My initial plan was to cobble something together but a chance perusal of the photos of a 1997 ST1100 Pan European that I renovated last year made me take a closer look at the locking mechanism on the ST's panniers. turns out they are identical and I managed to get a new catch from Honda spares guru David Silver for £6 plus postage. Part number is 81223MT3000 - HOOK. LOCK. But it may also help in the search for 2nd hand replacement locks.
(http://i660.photobucket.com/albums/uu327/Fentible/DSCN0203.jpg)
(http://i660.photobucket.com/albums/uu327/Fentible/DSCN8793.jpg)
(http://i660.photobucket.com/albums/uu327/Fentible/DSCN8794.jpg)
(http://i660.photobucket.com/albums/uu327/Fentible/DSCN8825.jpg)
(http://i660.photobucket.com/albums/uu327/Fentible/DSCN8827.jpg)